1. Field of the Invention
This invention relates to a secondary battery and an anode for a secondary battery.
2. Description of the Prior Art
As mobile terminals such as a cellular phone and a note-type personal computer have become widespread, a battery as their power source has become more important. Such a battery must be small and light-weight while having a higher capacity and must exhibit a property that it is tolerant to degradation due to repetitive discharging and charging.
Lithium metal is sometimes used as an anode in the light of its higher energy density and light-weight. However, as a charge-discharge cycle is repeated, needle crystals (dendrites) are deposited on a lithium surface. Finally, these crystals penetrate a separator to cause internal short-circuit, leading to a reduced battery life. When using a carbon material capable of occluding and releasing lithium ions as an anode, precipitation of needle crystals is not observed and a charge-discharge cycle can be successfully repeated. The carbon material may have a capacity smaller by about one order than lithium metal.
There have been, therefore, many attempts for improving an anode capacity.
JP-A 9-259868 has disclosed that metal powder such as copper, chromium and titanium incapable of forming an alloy with an alkali metal may be added to an anode to improve conductivity, reduce cycle degradation and improve an efficiency of a carbon material, and that conductivity and a capacity can be improved by using a carbon material supporting fine powder of a metal such as aluminum, lead and silver capable of forming an alloy with an alkali metal.
JP-A 2000-90916 has disclosed an anode active material in which powders of a metallic material (reduced material) made by heating ultra-fine particles of, for example, silica, alumino-silica, tin oxide and a composite metal oxide of tin oxide and antimony oxide are coated with a carbonaceous material. JP-A 10-3920 has disclosed an anode active material comprising fine particles which is made of at least one element selected from Mg, Al, Si, Ca, Sn and Pb and on whose surface a carbonaceous material layer is formed. It has been described that such an anode active material may be used to prepare a secondary battery with a higher capacity which is tolerant to cycle degradation.
Domestic re-publication of PCT international publication WO 96/33519 has disclosed the use of an amorphous oxide comprising at least one functional element selected from Sn, Mn, Fe, Pb and Ge as an anode material. It has been described that such an anode material may be used to prepare a safe non-aqueous secondary battery exhibiting a higher discharge operating voltage, an improved discharge capacity and excellent cycle properties.
JP-A 5-234583 has suggested that a carbon material coated with aluminum is used as an anode material for inhibiting rapid degradation of cycle properties caused when using an organic solvent with higher solvation force as an electrolyte. It can prevent intercalation between carbons while lithium ions are solvated, to prevent a carbon layer from being damaged and allow rapid degradation of cycle properties to be inhibited.
The prior art as described above has the following problems.
In the above technique described in JP-A 9-259868 that metal powder is contained in an anode or supported carbon material, metal particles cannot be evenly dispersed in the carbon material. Therefore, a metal is apt to be localized in an anode, so that repeating a charge-discharge cycle may cause localization of an electric field or peeling from a current collector. Difficulty of even distribution of metal particles would be due to difference in powder properties between the metal and the carbon material.
In a technique disclosed in JP-As 2000-90916 and 10-3920 that metal particles are coated with a carbonaceous material, uneven metal distribution is microscopically inevitable, leading to localization of an electric field. It is, therefore, difficult to maintain higher level of cycle properties.
These conventional techniques commonly have a problem that a high operating voltage cannot be obtained because when mixing a metal with a carbon material, a plateau peculiar to a metal is formed at a higher voltage than carbon in a discharge curve, leading to a lower operating voltage than that obtained when an anode is made of carbon alone. A lithium secondary battery has a predetermined lower limit voltage, depending on its application. Therefore, as an operating voltage is reduced, an available range becomes narrower. As a result, a capacity cannot be increased in a range where a battery is actually used.
The technique disclosed in Domestic re-publication of PCT international publication WO96/33519 also has the problem of an operating voltage. We have evaluated a battery using, as an anode, an amorphous metal-oxide represented by SnBxPyOx where x is 0.4 to 0.6 and y is 0.6 to 0.4 disclosed the above publication, and have found that it exhibits a lower operating voltage than that in a carbon anode and that a lower discharge current is required to achieve a sufficiently high capacity. Furthermore, the use of the anode material leads to increase in a weight, leaving room for improvement.
The technique disclosed in JP-A 5-234583 using aluminum as an anode material has a problem that as the cycle is repeated, a capacity is rapidly reduced, probably because electric field convergence to aluminum may cause, e.g., peeling in an electrode and aluminum reacts with water present in an electrolyte to form a thin insulating film on an aluminum surface.
The above conventional techniques cannot maintain a sufficiently high charge-discharge efficiency during long-term use, also leaving room for improvement.